The activated sludge process is currently used to treat industrial wastewaters containing pollutants such as organic components, for example, wastewaters discharged from iron works, steel works, gas works, and chemical plants petroleum wastewaters, pulp mill wastewaters, wastewaters from agricultural or marine product processing plants, as well as city sewage. Since different wastewaters have different compositions and cannot be easily treated by the activated sludge process under the same conditions, activated sludge treatments of these wastewaters are operated on the basis of empirical data. In particular, wastewaters discharged from coke ovens and petroleum refineries contain materials that inhibit the activity of bacteria, so if they are treated by the conventional activated sludge process, the efficiency of the influent biodegradation drops and the quality of the effluent is decreased. In most cases, the cause of this decrease in the efficiency of biodegradation is unknown and there is no definite means for restoring the normal biodegradation other than by letting the bacteria (activated sludge) revive themselves, which usually takes as long as at least one to six months.
A version of the activated sludge process that controls the conditions for treatment on the basis of the activity of the sludge, rather than empirical data is described in Japanese Patent Publication No. 27096/81 According to this process, the activity of the activated sludge is evaluated by measuring the oxidation-reduction potential (ORP) of the sludge (including return sludge) in the aeration tank. In order to maintain the sludge in an active state, various parameters (especially, the dilution rate of the sludge, aeration volume and the amount of return sludge) are controlled so that the value of ORP is held within the proper range. However, even this method involves difficulty in preventing a decrease in the biodegradation efficiency from occurring in the treatment of industrial wastewaters containing organic conponents and materials that inhibit the activity of bacteria. Once the biodegradation of the organic conponents is inhibited, maintaining the ORP within the proper range by controlling the aeration volume and other parameters is no longer effective in restoring the normal level of degradation within a short time.
In the activated sludge treatment of the industrial wastewaters listed above, phosphorus-containing compounds, especially inorganic phosphoric acid compounds, such as phosphoric acid and ammonium phosphate are commonly used as nutrients for bacteria. However because of the low availability of the inorganic phosphoric acid compounds to bacteria, the effluent obtained by process of the activated sludge treatment of industrial wastewaters contains unused phosphorus-containing compounds in a considerable amount.
As described above, when industrial wastewaters, especially those containing organic components and materials that inhibit the activity of bacteria, are treated by the conventional activated sludge process, a drop in the efficiency of biodegradation often occurs, and once it occurs, a prolonged period is necessary for restoring the normal biodegradation. Furthermore, the use of a large quantity of phosphorus-containing compounds, for example, inorganic phosphoric acid compounds, results in the production of effluent liquors containing a high proportion of these compounds. Therefore, a new process of activated sludge treatment free from these problems of the conventional method has been needed.